Wave shaping circuit



July 9, i945 R. SANDERS, JR

WAVE SHAPING CIRCUIT Filed July 25,' 1944 INVENTOR.

Patented July 9, 1946 WAVE CIRCUIT Royden C. Sanders,

Jr., Hightstown, N. J., as-

signor to Radio Corporation of America, a corporation of Delaware Application `Fully 25, 1944, Serial No. 546,537

6 Claims.

This invention relates to wave shaping circuits, and more particularly to improvements in the invention described and claimed broadly in U. S. application Ser. No. 512,153, iiledNovember 29, 1943, by Irving Wolff, and entitled Vibratory mechanical systems. The present invention is disclosed but not claimed specifically in said Wolff application.

The principal object of the instant invention is to provide an improved method of and means for producing wave shapes of the type required in the practice of the invention disclosed in said Wolfi application.

Another object is to provide an improved method of and means'for cyclically frequency modulating a radio transmitter in such manner that the product of bandwidth swept times the modulation frequency is independent of the modulating frequency.

'I'he invention will be described with reference to the accompanying drawing, of which Figure 1 is a schematic circuit diagram of the invention as applied to a frequency modulator system; Figure 2 is a graph of a square wave voltage employed in the operation of the system of Figure mittedV frequency. in megacycles per second per second. The distance sensitivity, in beat frequency cycles per foot of distance is-thus directly proportional to the rate of change of transmitter frequency Bfm.

1; Figures 3 and 4 are graphs of voltages derived from said square wave voltage in the operatin of the system of Figure 1 and Figure 5 is a graph of the wave applied to the actuating coil of the vibrating capacitor unit of the system of Figure 1.

One important application of the invention is in modulation in radio distance measuring devices, such as FM altimeters and the like. FM altimeters are well known, such altimeters being described in Bentley Patent 2,011,302 land in Espenschied Patent 2,045,071. In these systems a radio Wave that is radiated to a reflecting surface or object is cyclically frequency modulated. In a receiver which is located close to the point of radiation, the reflected signal is picked up and mixed or heterodyned with some of the frequency modulated signal received' directly from the transmitter. The resulting difference frequency is a measure of the distance from the transmitter to the reflecting surface or object, since this frequency is determined by the time required for the radiated signal to reach the reflecting object and return to the receiver.

The beat, or difference frequency, is

where B is the band width-swept, in megacycles per second, fm is the modulating frequency, in, cycles per second, and d is the distance in feet.

Practical experience has shown it to be desirable to employ a vibrating type modulating unit, preferablya capacitor device in which the capacity could be varied cyclically to vary correspondingly the transmitter frequency.- A coll driven diaphragm unit of this type is described in copending U. S. application, Ser. No. 471,003, filed January 1, 1943, by S. V. Perry, and entitled Capacity modulator unit. Generally the variation of capacity with respect to time need not be linear, or in accordance with any other` particular law, as long as it is cyclical. However, certain FM distance and speed measuring systems do require accurately linear triangular wave frequency modulation in order to perform properly ti. e

functions for which they are intended. While such modulation is obtainable with reactance tube modulators and the like, it is highly desirable, particularly in aircraft and other mobile applications, to retain the advantages of light weight, compactness, and simple construction characteristic of the vibrating diaphragm type unit. This choice, however, presents the problem of driving the diaphragm, with its physical properties of mass, resilience, and friction, so as to provide truly linear variation of capacitance with time.

Referto Figurel. A variable capacitor unit I of the type described in the aforementioned Perry application is provided with a stationary plate 3V coupled to the tuned circuit of an oscillator, not shown. 'A diaphragm 5 constitutes the movable capacitor electrode, which may be driven by suitable means, such as an electrodynamic drive. The drive is lrepresented as a Winding 7. Energy for the driving-winding 1 is provided by a modulator amplifier 9, through a coupling transformer II designed to match the output im pedance of the amplifier to the impedance of the winding 1. l l

The input circuit of the amplifier 9 includes a wave shape modification network I3, described more fully hereinafter and is connectedto yan intermediate tap on a voltage divider I4. One end ofthe voltage dividers is connected to the positive terminal B+ of theanode, potential supply (not shown) and the other end is connected through a switch I5 to ground. A battery. I1 is connected through 'a switch I9 toa motor 2i, preferably the motor section of a. conventional dynamotor used to supply anode potential for the amplier 9 and other equipment. The shaft of the motor 2| carries, or is mechanically coupled to a cam 23 for periodically operating the switch I to produce square wave impulses which are modied by the network I3, amplified by the amplifier 9` and applied to the actuating winding 1 of the variable capacitor device II.

The network I3 includes an integrating section 25, a differentiating section 25, and a combining section comprising series connected resistors 29. 3|, 33 and 35. The resistors 29, 3|. 33 and 35 also function as a load or termination for the differentiating section 21. 'I'he integrating section 25 comprises a resistor 31 and a capacitor 39 connected in series between the switch I 5 and ground. .The values of .the resistor 31 and the capacitor 39 are selected so that their RC product, or time constant, is long with respect to the period of one cycle of operation of the switch I5. A suitable modulation frequency is approximately 110 cycles per second. A coupling capacitor 4I is connected from the junction between the resistor 31 and the capacitor 39 to a point on the combining section between the resistors 3| and 33.

The differentiating network 21 includes a series capacitor 43 having a value with respect to that of the total resistance of the resistors 29, 3|, 33 and such that the RC product is small as compared to the period of operation of the switch I5. Also included in the network 21 is a series resistor 45 and a shunt capacitor 41, connected like the resistor 31 and the capacitor 39 of the integrating network. The function of these elements is described below.

A resistor 49 is connected between the point B+ and the junction between the resistors 33 and 35. The resistors 49 and 35 function as a voltage divider from which a positive bias voltage is applied to the control grid circuit of the electron discharge tube 5| of the amplifier 9. A selfbias resistor 53 is provided in the cathode circuit of the tube 5 I.

In the operation of the above-described system, the switch l5, whenclosed, connects the voltage divider I4 between the point B+ and ground. The .voltage at the tap of the voltage divider I4 is, therefore, somewhere between zero and the B+ potential, having a value dependent upon the adjustment of the tap. When the switch I5 is open, the full B+ potential is applied to the tap through the lower portion of the voltage divider I4. Thus as the switch I5 is opened and closed, the voltage applied to the network I3 is cyclically and discontinuously alternated between two definite values, providing a square wave form as illustrated in Figure 2.

As the switch I5 opens, the capacitor 39 starts to charge through the resistor 31. The increase of voltage across the capacitor 39 is substantially linear with respect to time during the period the switch I5 is open. When the switch I5 closes, the capacitor 39 starts to discharge through the resistor 31, since the voltage across it is higher than the voltage at the tap of the voltage divider I4. The decrease of voltage across the capacitor is also substantially linear with respect to time because the RC product of the resistor 31 and the capacitor 39 is large in relation to the period of operation of the switch I5. Accordingly the voltage across the capacitor 39 varies as shown Figure 3, constituting a linear It should be noted that the 3 is determined only 75 by the graph of triangular wave. slope of the wave of Figure by the amplitude of the square wave voltage and the constants of the integrating circuit (values of capacitor 39 and resistor 3l) and is substantially independent, throughout the operating range, of the square wave frequency. Thus, if the speed oi the motor 2|V decreases, decreasing the frequency of operation of the switch I5, the amplitude of the triangular wave output of the integrating circuit will increase correspondingly, since the capacitor 39 has correspondingly longer periods of charge and discharge. The slope. l0r rate of change of voltage, of the triangular wave thus remains constant.

The graph of Figure 3 is also representative of the form of the desired motion of the diaphragm 5 of the modulator unit I as a function of time. However, if the voltage across the .capacitor 39 were applied to the actuating coil 1 without modication of wave shape, the motion of the diaphragm would not be a linear function of time, owing to inertia.

At each extreme oi its excursion, the diaphragm tends to continue moving in the same direction after the driving force reverses, lagging the actuating current both during deceleration and acceleration and thereby producing a distorted wave of capacitance variation. In the practice of the instant invention. this effort is counteracted by predistorting the driving force by the addition of a sharp impulse at each reversal to overcome the momentum tending to cause continued motion in one direction and ,supply an opposite momentum to start motion in the reverse direction.

A preferred method of producing said impulses is by time differentiation of the square wave voltage of Figure 2, by the action of the capacitor 43 and the series resistors 23, 3|, 33 and 35. Neglecting momentarily the eilect of the resistor 45 and the capacitor 41, the capacitor 43, being relatively small, charges fully to the peak value of the applied square wave almost instantaneously upon opening of the switch l5. The short pulse of charging current, flowing through the resistors 29, 3|, 33 and 35, causes a similar pulse of voltage atthe input circuit of the amplifier 9. Upon closure of the switch I5, the capacitor 43 discharged through the same circuit, providing a voltage pulse of opposite polarity to the charging pulse. The pulse shape depends primarily upon the slopes of the leading and trailing edges of the square wave input to the capacitor 43. It is found in practice that the leading edge of the pulse may be too sharp to provide the desired .operation of the system. The resistor 45 and the capacitor 41 are included to decrease the slopes of the edges of the square wave voltage before application to the capacitor 43, in order to reduce the slope of the leading edges of the derived pulses. The resistor 45 and capacitor 41 operate in the same manner as the resistor 31 and thel capacitor 39 of the integrating section 25, but to a much smaller extent.

The triangular wave voltage from the integrating section is applied through the capacitor 4| to the resistors 33 and 35. and the pulse train from the differentiating section appears across the resistors 29, 3l, 33 and 35 as explained above. The resultant voltage at the tap of the resistor 29 contains components comprising both the triangular wave and the pulse train. The proportions of these components to each other is determined by the position of the tap. The input to the control grid of the amplifier tube 5| is of the form illustrated by the graph of Figure 5.

The ampliiier output .wave is of the same shape but greater amplitude.

The relative amplitudes of the triangular wave and pulse components are adjusted by means of the voltage divider 29 so that the pulses just reverse the momentum of the diaphragm at each end of its excursion, as described above. The amplitude of vibration of the diaphragm 5 may be adjusted by means of the voltage divider id, which controls simultaneously the amplitudes of .both components of the wave of Figure 5. As 'pointed out above, the amplitude of the triangular wave voltage varies inversely with variation in the modulating frequency. The amplitude of vibration of the diaphragm 5 varies likewise, so that the range oi variationln oscillator tuning, B, is inversely proportional to the modulating frequency fm. Thus the product Bim remains constant, and the distance sensitivity is independent of the modulating frequency im. f

The invention is described as an improved method of and means for providing linear triangular wave capacity variation for FM radio distance measuring systems with a diaphragm type vibrating capacitor modulator. A square wave voltage is produced by means of a periodic switch and a source of D.C.modied by an integrating network to triangular form, and applied to the actuating magnet of the capacitor unit. To overcome the enect of the inertia ofv the diaphragm upon linearity of operation oi the modulator, a pulse train is derived from the square wave and added alsebraically to the triangular wave, to onset the momentum at the end of each vibration of the diaphragm and supply opposite momentum at the beginning oi each vibration.

I claim as my invention:

l. In a modulator system including a vibratory variable capacitor with an electrical actuator, a system for energizing said actuator, including a source of square wave voltage, means for integrating said voltage to provide a voltage of triangular wave form, means for dinerentiating said voltage to provide a voltage pulse train of alternating polarities, a network for combining said triangular wave voltage and said pulse voltage, said including a vibratory variable capacitor provided with an electrical actuator, a system for energizing said actuator, including a source oi square wave voltage, a voltage integrating network andy a voltage differentiating network, both connected tioned resistor with said dinerentiating capacitor to square wave source, a voltage combiningcircuit including at least one resistor connected to one of said networks and at least one .other resistor connected between said networks,- an adcapacitor connected in high pass iilter conflguration, both of said networks being connected to said square wave source, a voltage combining circuit including at least one resistor connected across the output of one oi! said networks and at least one other resistor connected between one output terminal of said dinerentiating network and one output terminal of said integrating network, an adjustable tap on said latter resistor, and means i'or energizing saidactuator in respouse to the voltage between said tap and another point 'in saidcombining circuit.

` 4. A wave shaping circuitl for radio reilection modulator systems, comprising a pair of input terminals, a pair of output terminals, an output circuit comprising a plurality of resistors connected in series between said output terminalsra resistor and a storage capacitor connected lin series between the rst of said input terminals and the second of said output terminals, a couplingcapacitor connected from the junction of said .storage capacitor and said last mentioned resistor to an 'intermediate point in said output circuit, and a dinerentiating capacitor' connected from said first input terminal to the rst of said outputterminals. f'

5. The invention as set forth in claim 4, including a further resistor in series said dinerentiating capacitor, and a lfurther storage capaitor connected from the Junction of said last mento said second output terminal.

6. The invention as set forth in claim 4, wherein one of said series connected resistors oi' said output circuit includes an adjustable tap whereby the enects ot sai'ddinerentiating capacitor and said storage capacitor may be regulated dii'- y ferentially. g y

` ROYDEN C. SANDERS, JR. 

